The purification of edible oils includes various processing steps to remove substances which are detrimental to the quality of these oils. One of these steps is water vapor distillation, generally performed at temperatures of 180.degree. C. to 220.degree. C. and pressures of 4.6 to 15.2 mm Hg, in which volatiles, colored components and compounds which are odiferous and cause an unpleasant taste are separated from the oils. As the water vapor condenses, a liquid phase separates which is insoluble in water and essentially consists of free fatty acids, mono-, di- and triglycerides, tocopherols, and sterols. On cooling, this phase forms a material with a waxy consistency which is called vegetable oil distillate, or "VOD". A typical composition for soybean oil VOD is about 20% sterols (all percentages by weight), 10% tocopherols, 20% free fatty acids, and the remainder primarily mono-, di- and triglycerides. Among these components, the tocopherols, which exist in .alpha.-, .beta.-, .gamma.-and .delta.-forms, are of value because of their activity as vitamin E, a well-known anti-oxidizing agent. As a result, commercial processes exist for the recovery of tocopherols from VOD as well as from naturally occurring substances which are similar in composition to VOD, such as palm oil.
Prior art processes for separating tocopherols from VOD include esterification, saponification, direct solvent extraction, ion exchange resin chromatography, and direct distillation. Saponification is the preferred method since it produces alkali metal soap which, due to its insolubility in the solvent used in the process, can be separated from the dissolved tocopherols, thereby permitting recovery of the tocopherols in a form relative free from fatty acids and glycerides. The processes themselves are costly, however, and tocopherols are produced in low yield.
Of the saponification processes, the lime saponification process is the most widely used. Hickman, U.S. Pat. No. 2,349,270, discloses the use of slaked lime for the saponification followed by a granulation of the resulting lime soap mass and extraction of the tocopherols from the granulated material with ethyl ether. Andrews, U.S. Pat. No. 2,263,550, discloses saponification of VOD with sodium hydroxide, followed by metathesis (ion exchange) with calcium chloride to convert the sodium soaps to calcium soaps, from which the tocopherols and other unsaponifiable matter are then extracted with acetone.
The disadvantages of each of these processes is that the calcium soap is formed in a wide particle size distribution, ranging from fine particles to lumps. The result is a soap mass which is lumpy in form and from which the unsaponificable matter is difficult to extract. To permit the extraction to take place, the soap mass must be ground into particulate form, a process which entails a substantial capital investment. Even then, solvent consumption is high and the recovery of tocopoherols and other useful unsaponificable matter such as sterols is low.
Grinding is avoided in the process disclosed by Brown, et al., U.S. Pat. No. 3,108,120, which uses calcium silicate as a powdering agent in combination with acetone to facilitate the separation of the soluble tocopherols and sterols from the insoluble soap mass. Unfortunately, this process requires a large amount of powdering agent which remains in the soap mass, and the effectiveness of the powdering agent is diminished if the moisture content of the soap mass is too high.
Other disclosures of potential relevance to this invention are the disclosures of Rossouw, et al., J. Appl. Chem. 2:335-8 (1952), and Senda, et al., U.S. Pat. No. 3,983,147, both of which address the saponification of wool wax. The potential relevance is the description of saponification itself, and in the case of Senda, et al., the metathesis performed subsequent to the saponification. Wool wax however is not believed to contain tocopherols.
The problems cited above as well as others encountered in tocopherol recovery are avoided in the present invention, which addresses the need for a simple, cost efficient, high yield process for the recovery of tocopherols from vegetable oil distillates and other similar sources.